1. Technical Field
The present invention relates to a device for use with an internal combustion engine, particularly a lean burning engine. Furthermore, the invention is intended for use with the exhaust system of an internal combustion engine, with the exhaust system having a NOX absorber.
2. Background Information
Vehicles operating with an internal combustion engine must meet the general requirement of low level of harmful substances in the exhaust gas coming from the engine. These substances are predominantly compounds such as nitrogen oxides (NOX), hydrocarbons (HC) and carbon monoxide (CO). In the case of modern internal combustion engines, the exhaust gas is normally purified with the aid of a catalytic converter, which is part of the exhaust system and through which the exhaust gas is passed. Known xe2x80x9cthree-way catalystsxe2x80x9d remove the predominant part of the above harmful compounds through catalytic reactions. In order that the catalyst gives the highest possible degree of purification with respect to NOX, HC and CO, the engine is run on a stoichiometric mixture of air and fuel, i.e., one with a lambda value (xcex) of 1 in most operating modes.
Although modern three-way catalysts enable a very high degree of purification thereby greatly reducing the emission of harmful compounds into the atmosphere, there is a continual need for further reductions in the emission of harmful substances. This is in part due to the increasingly rigorous legislation introduced in various countries, requiring an extremely low emission of NOX, CO and HC compounds.
Furthermore, it is a general requirement that vehicles have the lowest possible fuel consumption. This has recently led to the development of engines with a new type of combustion chamber for the cylinders of the engine, making it possible to run the engine on ever leaner fuel mixtures, i.e., on mixtures with a lambda value (xcex) greater than 1. Such engines are generally called xe2x80x9clean-burnxe2x80x9d engines. A direct-injection (or xe2x80x9cDIxe2x80x9d) engine, which is a spark-ignition engine with direct injection, operates in a xe2x80x9cstratifiedxe2x80x9d manner. Here, fuel introduced into the engine""s combustion chamber is concentrated to a great extent at the sparking plugs. When working in certain modes, such as under a low or medium engine loading, these engines are able to run on very lean air/fuel mixtures with a lambda value of the order of magnitude of 4. This considerably increases the fuel economy for these types of engines. A direct-injection engine can also run in a xe2x80x9chomogeneousxe2x80x9d manner in certain modes of operation, mainly when driven under a high engine loading. The homogeneous operation corresponds to a stoichiometric (i.e., a relatively rich) air/fuel mixture being fed into the engine.
Since, in certain modes of operation, a direct-injection engine works on a very lean air/fuel mixture, the exhaust gas mixture that consequently flows through the three-way catalyst is also lean. As such, the three-way catalyst cannot reduce the NOX compounds in the exhaust gas since it is designed to give an optimum purification in the case of a stoichiometric mixture. To remedy this, the ordinary three-way catalyst is combined with a nitrogen oxide adsorber, also called a NOX adsorber or NOX trap. The purpose of this arrangement is to adsorb the NOX compounds, e.g., from the exhaust gas of an internal combustion engine. These NOX adsorbers can thus be installed and used in conjunction with a conventional three-way catalyst.
A NOX adsorber may be arranged as a separate unit located upstream of the ordinary three-way catalyst, or alternatively it may be integrated with the three-way catalyst, i.e., combined with the catalytic material in the three-way catalyst. The NOX adsorber takes up (adsorbs) NOX compounds present in the exhaust gas of an engine when it is run on a lean air/fuel mixture, and gives off (desorbs) the NOX compounds when the engine is being run on a rich air/fuel mixture for a certain time.
Furthermore, the NOX adsorber can only adsorb the NOX compounds up to a certain limit, i.e., it gradually xe2x80x9cgets fullxe2x80x9d with its adsorption capacity reaching its limit. When this happens, the NOX adsorber has to be regenerated, which means that it must be made to desorb and release accumulated NOX compounds. If there is a conventional three-way catalyst downstream of the NOX adsorber, these desorbed NOX compounds can be eliminated in the three-way catalyst, provided that the latter has reached its operating temperature.
The NOX adsorber can be regenerated by making the exhaust gas that passes through it relatively rich for a certain time. This is achieved by running the engine on a relatively rich air/fuel mixture for a short time, e.g., for a few seconds. In doing so, the NOX adsorber is depleted so that it can again adsorb NOX compounds for a certain time before another regeneration is needed. U.S. Pat. No. 5,461,857 describes achieving regeneration by controlling the concentration of air in the exhaust gas mixture flowing through the NOX adsorber.
A NOX adsorber is designed to operate at a certain temperature, which, in turn, depends on the mode of operation of the engine at the time. During stratified operation, i.e., in the case of a lean air/fuel mixture, the temperature of exhaust gas flowing through the NOX adsorber should be about 200 to about 500xc2x0 C. if operation of the adsorber is to be optimized. Furthermore, it is a general requirement that the exhaust gas temperature should not exceed about 800xc2x0 C. because the NOX adsorber may be damaged at higher temperatures.
A phenomenon that occurs in a NOX adsorber is that sulfur compounds, such as sulfur dioxide (SO2), present in exhaust gas flowing through the NOX adsorber, form a deposit on the active material of the NOX adsorber. Having this deposit on it prevents the NOX adsorber from adsorbing NOX compounds. The sulfur compounds come from the fuel and vary with the quality of the fuel. As a result of this a sulfur deposit, the adsorption capacity of the NOX adsorber progressively decreases with time.
To remedy the problem of this sulfur deposit, the NOX adsorber is regenerated at set intervals in order to free the absorber from these compounds. As known in the art, such sulfur regeneration can be carried out by running the engine for a certain time wherein a rich exhaust gas with a lambda value of less than 1 is produced while the exhaust gas has a relatively high temperature, specifically one in excess of about 650xc2x0 C. In this manner, sulfur compounds are desorbed, i.e., they are discharged from the NOX adsorber. As known in the art, this sulfur regeneration is preferably done at intervals based upon how much of the NOX storage capacity of the NOX adsorber has been lost. The loss of storage capacity is estimated from the sulfur content of the fuel and the fuel consumption of the vehicle in question.
However, a problem that occurs in the prior art with the sulfur regeneration is that it is difficult to reconcile the desired exhaust gas temperature when running a lean fuel mixture. This desired gas temperature for a lean fuel mixture is about 200 to about 500xc2x0 C. However, in order to carry out sulfur regeneration, it should be at least about 650xc2x0 C. in the NOX adsorber. This problem can be conventionally solved by raising the exhaust gas temperature during sulfur regeneration, e.g., by delaying the ignition timing for the cylinders of the engine. However, this measure is not sufficient to raise the exhaust gas temperature to the required value if the vehicle in question is never run under a high engine loading, which happens with certain types of drivers and certain types of driving situations.
Accordingly, there is a need to resolve the conflicting requirements of a high temperature of at least about 650xc2x0 C. needed for sulfur regeneration, and a relatively low temperature of about 200-500xc2x0 C. that occurs when running a lean mixture and sulfur regeneration is needed. Further, the temperature must stay below about 800xc2x0 C. under all conditions in order to avoid damaging the NOX adsorber.
This problem may be solved by introducing a bypass construction into the exhaust system whereby exhaust gas is passed along different routes through the exhaust system depending on the temperature. However, this solution requires a costly valve arrangement, and such a separate valve arrangement in the exhaust system might also jeopardize the reliability of the latter.
The present invention provides an improved device for removing harmful substances emitted by an internal combustion engine. In particular, the invention provides a device that ensures the right working temperature of an NOX adsorber connected to an internal combustion engine. This is achieved with the aid of a device having a heat exchanger that is fitted upstream of a NOX-adsorber catalytic converter used to adapt the temperature of the exhaust gas from the engine to the operating state of the catalytic converter.
The present invention relates to a device that comprises a control system or means for generating an air/fuel mixture for the cylinders of the engine, an exhaust system connected to the engine, and an NOX-adsorbing catalytic converter arranged in the exhaust system. The invention also comprises a heat exchanger that is fitted upstream of the catalytic converter, and which is used to adapt the temperature of the exhaust gas from the engine to the operating state of the catalytic converter at the time.
The present invention offers several advantages. First of all, using a heat exchanger in conjunction with a direct-injection engine makes it possible to adapt the temperature of the exhaust gas flowing through the catalytic converter to match the mode of operation of the engine at the time. It should be mentioned here that the device according to the invention is a xe2x80x9cpassive systemxe2x80x9d, which does not call for a separate valve arrangement or the like to effect this temperature adaptation. For the sulfur regeneration of the catalytic converter, an exhaust gas temperature that lies in the range of about 650 to about 800xc2x0 C. is needed for satisfactory operation. Moreover, the use of the device according to the invention does not mean that the back pressure in the exhaust system is less favorable than in the conventional exhaust system, which would reduce the torque. Furthermore, the device according to the invention can be fitted in a vehicle with relatively small available space and with a fully effective cooling surface area.